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What is the theoretical maximum speed of a rocket-powered aircraft in the atmosphere?
I read that the scram jet has a theoretical maximum speed of Mach 24. What is the theoretical maximum speed of a rocket-powered aircraft designed to operate in the atmosphere?

Why do you think such a value exists?  Given a big enough engine, there may well be no upper limit at all.

@abelenky: Not entirely true.  The speed of light is an absolute maximum, of course.  I'd also guess that there's a maximum acceleration you can get from a rocket exhaust, so you'd be limited by how long that acceleration takes you to get out of the atmosphere.  And of course you have practical considerations like how much fuel you can carry...

@jamesqf, the assumption is clearly that you accelerate in a way to stay in the atmosphere. And the thing with practical considerations is that they don't give you a theoretical limit, just a practical one.

@abelenky size of the engine alone doesn't buy you very much: a bigger engine will consume more fuel, which is heavy, requiring a bigger engine... it's the curse of the rocket equation. That still doesn't imply a theoretical hard limit though, just that it gets exponentially expensive to attain higher speeds.

The speed of light is not an actual physical limit. It is just the maximum speed that any object can appear to move with respect to another.

This would depend on the altitude, (or should I say, the air density).  At an arbitrarily high enough altitude, where the air is thin enough to be at whatever arbitrary density you choose, just about any speed is attainable. Or are you asking what is the maximum attainable indicated/calibrated airspeed?  I'd guess that would be based on the dynamic pressure that would cause the best possible insulator (think space Shuttle Heat Shield tiles), to melt or break apart from heat stress.

If a limit exists, that would be a practical limit not theoretical. Like if somehow you make a nuke rocket, e.g. set off a nuke to generate a tremendous pulse of thrust which powers an object of the size of a football, you could reach some insane speed I would imagine. And someone would actually try that someday.

@user3528438: However to achieve some velocity, some acceleration must occur, and this acceleration will be smaller if you have more fuel onboard. In practical, if I understand correctly this comment, the limit is twice the ejection velocity, unless some staging mechanism occurs to reduce the empty mass.The rocket equation is valid in space, without gravity, but in dense atmosphere, the drag created by the shock wave should also limit the velocity.

Even a sub-luminal rocket in level flight could destroy the planet. See XKCD Relativistic Baseball

Once the rocket reaches orbital velocity it would need to exert a downward component of thrust to avoid leaving the atmosphere.  That’s not a hard limit but a confounding factor.

The upper limit is given by the L/D of the rocket-powered vehicle and its thrust.
In order to follow the curvature of the Earth so it will stay within the atmosphere, the vehicle must create enough downforce by flying inverted so it can bend its flight path enough to not escape to outer space.
A practical altitude for flying this way would be dozens of kilometers up in the higher atmosphere, so drag and heat loads stay manageable. For the L/D I use the approximation given by Dietrich Küchemann (replace with a more recent one if handy):
$$\left(\frac{L}{D}\right)_{max}=\frac{4\cdot(Ma+3)}{Ma}$$
which would result in an L/D of 4 for Mach growing very large. Add to this the effect of gravity and your maximum downforce is the weight of the vehicle plus four times its rocket thrust. This force now has to balance the centrifugal force which results from flying at constant altitude:
$$m\cdot g + 4\cdot T = \frac{m\cdot v^2}{R_{Earth}+h}$$
where $R_{Earth}$ is the earth radius of approximately 6367 km and $h$ the flight altitude above ground. Now solve for flight speed v:
$$v=\sqrt{\left(g + 4\cdot\frac{T}{m}\right)\cdot\left(R_{Earth}+h\right)}$$
By flying in dense air close to the ground the lift force will be immense, but so will be the required thrust. Since flight altitude is small compared to the radius of the Earth, the equation can be simplified with a small loss in accuracy:
$$v=\sqrt{\left(g + 4\cdot\frac{T}{m}\right)\cdot6.400.000 m}$$
A more precise result would now add the effect of Earth's rotation, so flying eastwards along the equator will reduce flight speed by 464 m/s and add the same amount to the speed calculated above when flying westwards.

Using a modern Raptor engine with a 200 "marketing" TWR we get a bit over 220km/s. I'm not sure Kuechemann accounted for drag due to nuclear fusion :)

@Sanchises: Add a realistic amount of fuel and structure and the speed will be more realistic, too. However, it will always be a multiple of escape velocity so your unit of km/s is appropriate. But you are right, Küchemann's is only a crude approximation.

Could your proposed limit be exceeded simply by pointing the nose down to tilt the thrust vector downward, using thrust rather than aerodynamic lift to provide the required downforce?  I suppose not, as long as the achieved L/D is better than 1:1--

@quietflyer:Yes, but the efficiency of that is L/D lower than just flying in more dense air, so this only helps when L/D drops below 1, as you rightly point out.

In theory the max speed within the atmosphere is the speed that makes the air  resistance equal to the strenght of the material the rocket is build from.

Is this common knowledge? Do you have a source to reference?

Assuming we're talking about an aircraft, that must complete a trip between point A and  B, without destroying itself or its environment. If we also assume that we have the capacity of creating somehow the propulsion necessary to achieve any speed, then the limits would be related to:

Escape Velocity, we don't want the aircraft to leave Earth, thus, there's at least one upper bound equal to 40280 km/h.

Aerodynamic heating, fuselage's material must not melt nor break from higher temperature and air pressure.

Lockheed SR-71 Blackbird, which has been labelled as one of the fastest aircrafts ever, was made of titanium alloy, titanium alloy has a melting point around 1700 °C, even if you're using tungsten you're limited to its melting point of 3414 °C.

At high speeds, a shock wave forms, which deflects the air from the stagnation point and insulates the flight body from the atmosphere. This can affect the lifting ability of a flight surface to counteract its drag and subsequent free fall. This depends on Reynolds number.

I'm not an expert so I do not have the means to determine a number, but a good first order estimation can be made taking this two factors into account

@ymb1 thanks for the welcome, best source I could found online is https://history.nasa.gov/SP-60/ch-5.html
Shock waves are actually an issue for both supersonic and hypersonic flight

The fastest aircraft ever was the X-15, which was powered by a rocket engine, btw.

@PeterKämpf I'll edit my answer


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What happens if a pilot uses left rudder during a turn to the right?
What happens if the pilot turns to the right using ailerons but then simultaneously applies left rudder? I'm asking about a large aircraft e.g. Boeing 747-800 or Airbus A380, or any other aircraft. If there are any negative effects, what mechanical or other measures are in place to avoid this (besides strict training)?

What you're talking about is referred to a as a slip. See this Wikipedia article here

From what I read, the Sopwith Camel required left rudder for both left and right turns. =)

what mechanical measures are in place to avoid this?

The short answer is none.
Cross-controlling (the usual term for applying opposing aileron and rudder) is sometimes desirable.  Its greatest use in large aircraft is to align an airplane with the runway just before touchdown from a crabbed approach in a crosswind while keeping the upwind wing from rising. In other words, if you had a crosswind from the right and you were crabbed to the right to counteract it, just before touchdown you would apply left rudder to swing the nose left while applying right aileron to keep the right wing from coming up (or maybe put the right wing down a little).

What happens when the pilot makes a right aileron turn but then gives a left rudder turn simultaneously

I'm making the assumption that you're talking about a turn not as part of a crosswind landing, and I suspect that your assumption is that pilots of large aircraft normally use coordinated rudder input when they use the ailerons. However, that is not the case. Once the aircraft is off the ground and absent any need to counter asymmetrical thrust from a failed engine, pilots generally take their feet off the rudder pedals and do not put them back on until short final. Yaw dampers or the equivalent for fly-by-wire systems take care of any adverse yaw ailerons may produce. Thus a cross-control scenario for en route flying would mean the pilot would have to make not one, but two errors. They shouldn't have had their feet on the rudders in the first place. Then they shouldn't have cross controlled.
But let's say that, for whatever reason, during en route flight, a pilot deliberately cross-controls once and then releases the controls. Generally speaking, that would not be disastrous. However, if they held a cross-control condition or kept repeating it in sequence, that's another story and would depend on a number of other conditions, and you could certainly come up with scenarios that ended in the destruction of the aircraft. Although I'm not sure the fly-by-wire airplanes would permit you to do that, but I've never flown any.
If you were in a 747-100/200 en route, you could if you wanted give it left aileron and right rudder in proportions that matched. In other words, the turning force of the ailerons would be cancelled out by the turning force of the rudder. The airplane would keep going straight ahead. However, you would be in a sideslip, and you'd have to increase the wing's angle of attack to keep from descending, which would mean more drag. Plus, there would be all that additional drag from the rudder. In other words, you'd be burning a lot more fuel than you should. Plus, you'll get a call from the purser asking what the hell is going on because gravity is no longer perpendicular to the cabin floor.

From my Airbus manual:

Regardless of the airborne flight condition, aggressive, full or nearly full, opposite rudder pedal inputs must not be applied. Such inputs can lead to loads higher than the limit, and can result in structural damage or failure. The rudder travel limiter system is not designed to prevent structural damage or failure in the event of such rudder system inputs.

@Ulu83, but does that mean opposite to the aileron input? I think this was added after the American Airlines flight 587 accident, in which case it means opposite than previous rudder input, i.e. quick reversal.

Hello jan, correct, but I was trying to clarify that fly by wire aircraft do not offer protections here.

@Ulu83 When you're responding to a comment, if you press @ as the first  character, a pop up box will show which, when you type the first letter of the name of the person you're commenting to will display the name of commenters to the question starting with that letter. Then just click on the one you want (there's usually only one of course). The nice thing about this syntax is that whoever it is will be notified about the comment. When I composed this comment I pressed "@u" then clicked on your Ulu83, and which is why you will be notified of this comment.

This is just a sideslip.
It is a somewhat standard maneuver for steepening an approach without increasing airspeed, since it significantly decreases lift-to-drag ratio. It was famously used by the Gimli Glider, for example.

Sideslips are generally not used in large aircraft for that purpose.

A slideslip is used to correct angle. You're talking about a forward slip, which is used to steepening an approach. @Terry is right, forward-slips are generally not used by larger aircrafts.

The aircraft is now in uncoordinated flight and if you stalled, a spin could occurred.
What is a spin?  A spin is autorotation about the vertical axis and a shallow, rotating, downward path.
Some aircraft spin are prohibited because the pilot might not be able to recover and in time the aircraft impacts the ground and can cause total destruction of the aircraft and loss of life of the pilot and passengers.
Believe it or not, in the barn storming days of aviation pilots would be on top of the clouds and not be able to see the ground. They would actually put their airplanes into a spin and begin to descend in the clouds. The plan is to break out of the clouds and see the ground before it is to late
to recovery from the spin.
Now, in the modern Era of Aviation, there is electronic navigation to guide us in for a landing. This is called an instrument approached.
In some airplanes uncoordinated flight is used to land in crosswinds. The pilot uses the rudders to keep the nose of the aircraft lined up with the centerline of the runway and the ailerons for wind drift to keep the airplane over the runway. This is called a side slip. If the wind blew the airplane to the left of the runway, the pilot would apply right aileron and vice versa.
Another flight maneuver is called a crab and is also used for crosswind landings. The nose of the airplane is aimed into the wind direction to keep the ground path lined up with the runway. Wind from the left aim, the nose is aimed left and wind from the right, aim right. How much angle you use depend of the wind speed. In the flare, the wind correction angle is taken out and and the pilot lands the airplane.

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Can you turn a large airliner using only rudder input?
Is it possible to turn a large jet like the Boeing 747, Airbus A380 or other airliner using only the rudder, i.e. without using the ailerons on the wings?  Will it take longer to turn to a new heading? I find the tilting when ailerons (wing flaps) are involved is uncomfortable.

Duplicate of this question which is itself a duplicate of this question

«I find the tilting when ailerons (wing flaps) are involved is uncomfortable.» Besides the fact that flaps and ailerons are very different things, where did you experience this? In a well-executed (aileron) turn you should hardly notice anything.

Yes you can, and this is the backup mode for turning the A320 when the Fly By Wire has failed. It is not very comfortable though:

Changing direction using only the rudder creates sideways acceleration, not good for passengers with drinks in front of them
Since the rudder is located above the center of gravity, the aircraft leans the wrong way when rudder only is used, amplifying the sideways acceleration. A ship has the rudder below the center of buoyancy and leans into the turn when rudder is deflected.

Actually, I'm told by someone who should know that the difference between a ship and a boat is which way it heels over in a turn; a boat heels inward, a ship heels outward.  This is why submarines are always "boats" to sailors.

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At what temperatures would the gearing for geared turbofans be rendered unnecessary?
If geared turbofans were fitted with material science technologies or other engineering innovations that would allow them to operate at higher temperatures exceeding their current most optimum operating efficiency, at what point or temperatures would gearing be rendered unnecessary?

Temperature increases the efficiency of the turbine. Gearing increases the efficiency of the fan. Both are important, increasing one doesn't mean you can ignore the other.

The premise of the question is wrong.  Gearing has nothing to do with temperature limitation, rather speed limitation.  Turbines are most efficient when the run fast, fans are most efficient when they run slow.  In a normal engine, the fan and LPT are forced to run at the same speed.  So there is always a tradeoff.  Usually the LPT is forced to run much slower than optimal. For example, compare the PW1100G to the LEAP-1A.  The LEAP has a 7 stage LPT, but the PW1100G has only a 3 stage LPT.  In order words, because it is able to run faster, each stage of the PW1100G is about 2x more efficient than a LEAP LPT stage.  Therefore, it only needs half as many stages to do the same amount of work.  This is only a speed thing, nothing to do with temperatures.


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What is the sequence of ATC frequencies for final approach at JFK?
Recently I was listening to liveATC.net and tried to understand the sequence of operations for the voice communications.
However, it seems that there are a lot of frequencies for the approach procedure, and I got confused by their names and jobs. For example, the JFK terminal has two approaches (CAMRN 128.125 and ROBER 125.700), one Final approach (Final 132.400) that I don't really know what it is, and one two Tower frequencies (123.9 and 119.1). So the question is, (1) I think the tower will be covering the final approach but what is the "Final 132.400" frequency for? (2) when and where (e.g., distance from the airport) the aircraft will be handed over from one frequency to another? Does the tower frequencies only cover up to 5 nautical miles from the airport?

"Final 132.400" is final vectors (sometimes called 'final sector'), an intermediate stage between approach and tower. Whichever route an aircraft arrives, the relevant approach controller will hand it over to 'Final' for final vectoring onto the relevant final approach. Once the aircraft is established on that final approach (with proper spacing), it is handed over to tower.
I would recommend listening to Final first, whilst simultaneously watching a certain well-known flight radar website. It may take a few minutes to get 'in sync' with what's going on there, but you should soon get the gist of it. Then listen-in on the approach sector(s).


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What is the 3 click sound when the autopilot is disconnected on Airbus aircraft?
On Airbus aircraft what is the 3 click tone that sounds once autopilot is disconnected? I have tried researching on the internet but could not find anything

To inform the crew the A/P is now inactive? Disconnection may result from a fault, or an unintentional action.

Hi.  Welcome to Aviation.SE. I'm sorry, but we seem to have been suddenly invaded by downvotopaths. What's wrong with this question?.  +1 from me since I see this as a perfectly legitimate question and just to restore the natural balance of the stack-o-verse.

Are you looking for a recording of the sound?

I believe that sound was invented by Airbus specifically for signaling autopilot disconnect.

Sorry guys I think you misunderstood, I meant that one autopilot is disconnected there is a three click tone "bleep bleep bleep" after that there is another one it's like "bink bink bink"

Are you asking for sounds in a simulator (which one) or a real aircraft? Does that happen each time the A/P is disconnected manually and while the aircraft is within the regular flight envelop?

If you can include a link to a YouTube video and a marker of the sound you're describing, that would make the question much more clear.

https://youtu.be/XU3B8Qq1niY This video at 2.55

This  sound just after "approaching minimums" announcement (in your linked video) is the sound linked in the first comment. so "sorry guys I think you misunderstood..." has been and is still confusing.

Thanks for that link @mins. I've been looking for that ringtone

There are two separate aural warnings that you might be referring to: one is indeed called 'Triple Click' and consists of three brief clicks, but is emitted when the landing capability becomes degraded or the autopilot otherwise changes mode.
The warning generated upon autopilot disconnection, instead, is the so-called 'Cavalry Charge' and consists of three longer beeping sounds; it's either played once or repeated continuously (cases 1 and 2 below).
From a table in the Airbus A320 FCOM, DSC-31-10 P 4/6:

TRIPLE CLICK
Meaning: Landing capability downgrade or some cases of mode reversion.
Duration: 0.5 s (3 pulses)
CAVALRY CHARGE
Meaning:

A/P disconnection by takeover push-button.
A/P disconnection due to failure.

Duration:

1.5 s
Permanent

Cancellation:

Second push on takeover push-button.
Press Master Warning light or takeover push-button.

See this YouTube video (at 2:52) for a cavalry charge immediately followed by a triple click (thank you Jack).

https://youtu.be/XU3B8Qq1niY This video at 2.55

Any idea why they call that a "cavalry charge?" It doesn't really make me think of the standard bugle charge.

More generally, the Flight Crew Techniques Manual describes the TRIPLE CLICK as:

...an attention-getter, designed to draw the flight crew's attention to the FMA.

The FMA will show a mode change or reversion by putting a white box around the new mode and pulsing the associated FD bar.


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Why do propellers on aircraft have a slight offset angle?
Single-engine aircraft (one propeller on the 'nose')
Why do propellers on aircraft have a slight offset angle? I have picked up this information that the rotational axis of the propeller is not aligned with the roll axis of the aircraft, but I don't know why and by how much it is off. I also don't know the direction in which the propeller's rotation axis pointing (other than 'roughly' in roll axis direction).
Is the direction of the propeller's rotation axis important?
Does the direction of the propeller's rotation have any influence on it?
How big are these angles between aircraft roll axis and propeller rotation axis typically?
Is this concept only useful for propellers?

The direction of rotation of the propeller introduces an asymmetry which designers try to mitigate by adding more asymmetries. Specifically, a right-turning tractor propeller (clockwise from the pilot's viewpoint) will swirl the propeller wake in the same direction which adds a negative sideslip angle on the vertical tail (which sticks up into the upper half of the wake only). By pointing the axis of rotation of the propeller slightly to the right (right sidethrust), this sideslip angle can be reduced.
Other means of correction like an incidence on the vertical tail would not work equally well: Once the engine is throttled back, such an incidence would require to re-trim the aircraft. Still, siting in the prop wash, the effect scales with engine setting and helps to reduce the needed trim changes.
On multi-engined aircraft a sidethrust can help to reduce rudder input in engine-out cases. Here each wing engine points slightly away from the fuselage so its thrust will pull the aircraft into a helpful sideslip when the opposite engine has failed.
The propeller axis can also point slightly down in order to move the thrust line closer to the center of gravity, reducing thrust-related trim changes, and to reduce the P-factor effect (sideways shift of the thrust vector) at high angle of attack. This is called downthrust. One example would be the RR Griffon-engined version of the Supermarine Spitfire: When the new engine was installed, its lower thrust line required 2° more downthrust.
This is one of the few topics where the English version of Wikipedia stays schtum; only the German version has an article on it (Motorzug and Motorsturz).

Some planes have this designed into the engine mounts to offset the more pronounced left turning tendencies of larger engines.  "Large" in this case is a 400/500 series Lycoming or Continental.
Googling "Bonanza canted engine" will turn up more information than you could ever need and you can probably substitute "bonanza" out for any other model of common single engine airplane (Centurion, Skylane, Mirage, etc).
The angle is only a few degrees and the direction of rotation does decide to which side the engine is canted.  The "large" (285hp) engined Bonanzas going back to at least the S35 model have engines canted 2.0° down and 2.5° right of center.

Yes, but "why" is the question asked...

On American single engines planes, the engine rotates in a clockwise rotation as see by the pilots view.  Once airborne, this would have a tendency to "torque" roll the aircraft to the left.  To compensate, the pilot would apply right rudder to prevent the left torque rolling tendency.  Aircraft designers tend the offset the propeller thrust line a few degree to the right which in effect be giving right rudder.
There is also another matter to keep in mind, it is called P-factor.  P-factor refers to downward going prop (on the right) is facing a greater relative airspeed compared to the upward going prop.  This is because during a climb the aircraft is piched up around 10 degrees, thus the downward prop has greater airspeed going over it, when compared to the upward prop which is going opposite to the airplanes forward speed thus experiencing a relative drop in airspeed to the prop.
The net result is, during a climb,  the right Side of prop produces more thrust that the left side of the prop, thus also yawing the plane to the left.

The chosen answer already describes all of this...


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Can a plane help a second aircraft with landing gear issue mid-air?
My Father received his flight training in the Navy in 1941. He flew mostly Curtiss Wright SNC's (a model of the CW-22) and Grumman F3F's. The story I was told is hard to believe, so I'm hoping that some of you pilots out there (I'm not one) can tell me if this kind of stuff really happened.
While returning from a training run and preparing to land in an SNC, one landing gear would not go down. A second plane pulled alongside my Father's plane and with its wing tried to knock the landing gear loose! While in mid-air!
I find this hard to believe, but hey, you never know.
Many thanks for any insights!
td

There is a true tale of an F-86 pilot in Korea that kept a mate in the air after an engine failure from flak damage by nosing up to his tail pipe and pushing him along for some number of minutes, long enough to get the guy out over the sea where the Navy could rescue him.  The guy punched out and landed in the water, but drowned from getting entangled in the parachute lines as the rescue helicopter approached.

It's plausible, I don't know of any documented cases of it being done.

You’ve seen this video?

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What livery is on this F-5 airframe?
Markham Airport near Toronto used to have quite a few derelict airframes sitting on the airfield, with most sporting RCAF liveries but some coming from other countries such as Belgium. There is one aircraft, however, I've always been curious about - Pictured here in a photo uploaded to Google Maps in 2011:

Where is this livery from? I don't recognize it as belonging to any air force - Was it perhaps a special demo team livery?

You can easily get the answer searching by registration number: 116703

I flew out of that airport in the mid 70s.

That is a Northrop F-5/Canadair CF-5. Owned by the RCAF.

Around 1989 the aircraft was painted in this striking 419 Squadron color scheme for air show appearances (as was CF-5A 116740)

https://www.airhistory.net/photo/478325/116703

Any idea what happened to the vertical stabilizer?  It's missing from the OP's photo.  Either that, or they are using it to demonstrate some incredible new stealth technology. Perhaps it's just more advanced than previous attempts at invisible planes? https://www.youtube.com/watch?v=_mrWUrMK5d4 ;)

@AmazonDiesInDarkness I was wondering about that too - Maybe the unit kept it to display when the airframe was scrapped?

@Marsroverr That's a good guess. Perhaps it is hanging on someone's garage wall.  It would make an incredible dining room table too.


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Would there be any further restrictions on the Zapata Flyboard Air than current FAA regulations for ultralight aircraft?
We are seriously considering investing in Zapata Flyboard Air personal hovercraft.  We will use it on the east coast of USA Delaware, Maryland, New Jersey.  The Flyboard Air was designed to fit FAA guidelines for ultralight aircraft.
However this article covers how this vehicle has been banned from flight in France where Zapata is based.  While aviation regulations have always been more stringent in Europe, we don't want to risk our investment.  Would there be any additional regulations for this type of personal hovercraft over and above those already applicable to ultralight aircraft?
Would federal regulations override state and local laws that are bound to come?  How about Homeland security and privacy concerns?

It's jet powered, and multi-engine at that! This may require a type rating and unique maintenance requirements which may open it up to a few more regulations. If the idea is that you're investing in a recreational toy builder (like a jet ski or dirtbike company) you might be disappointed. A flyboard customer may need to take mandatory training (expensive) and adhere to maintenance requirements to stay legal.

@acpilot 103.7(b) says that ultralights are exempt from all pilot certification requirements

Huh.  Ok.  Rock-n-roll!!

Presumably the regulations could be changed if this device became popular and presented new problems, just as drone regulations have changed over the past few years. I don't know how anyone here could advise you on what actions the government might take in the future if they decide to make new rules.

That is one courageous dude!

There have been debate on how much this supposed restriction was a publicity stunt to attract media/politics or not (it was announced during election campaign time). Sadly, I couldn't find any authoritative article on the subject, but take it with a pinch of salt.

"this vehicle has been banned from flight in France": It seems it is subject to authorizations, but not prohibited a priori. Even without entering into safety questions, seeing people close to the board with fingers in their ears, seems to call for prohibiting use over populated areas.

Zapata was part of the July 14 (Bastille day) parade in Paris, along with other aerial  and land demonstrations.

If the Flyboard Air meets the definition of an ultralight from 14 CFR 103.1 then only part 103 regulations would apply. 103.7 says that ultralights are exempt from airworthiness, registration and pilot certification requirements, and 91.1(e) says that general flight regulations don't apply to ultralights.
Based on the specifications on Zapata's website, there are two models: the Flyboard Air EXP and the Flyboard Air UL. The EXP doesn't meet the part 103 requirements (its fuel load and top speed are too high) but Zapata says the UL was designed to comply with part 103 and the quoted specifications do seem to be OK.
Assuming that you're fine with the UL model there are still two points in part 103 that might be an issue for you. First, the stall speed requirement in (e)(4):

(4) Has a power-off stall speed which does not exceed 24 knots
  calibrated airspeed.

I don't know how the FAA would define a "stall" for a Flyboard and their detailed guidance on part 103 (dated, but seems to be still current) doesn't clarify it. It does say that a "recognized technical standards committee" can confirm that the requirement has been met, but I don't know who that means. You might want to ask Zapata how the UL complies with this requirement (and how they documented it).
Second, you can't use an ultralight in some commercial activities. The FAA document lists activities that are allowed (e.g. airshow participation) and others that aren't (e.g. aerial advertising). If you plan to use it only for private recreation then that doesn't matter, of course.
Finally, note that the FAA's document says that if you "encounter" an FAA field inspector for any reason it's up to you to prove that the ultralight meets the definition in 103.1. If you can't, they'll consider it as an aircraft for compliance/enforcement purposes. Before making your investment, I'd make sure that you're confident you can prove it really is a part 103 ultralight. If you aren't sure that you have the right information to do that, you could contact your local FSDO and ask them to review whatever you have and tell you if they would be satisfied.
(This answer is only about federal aviation regulations. If the Flyboard causes a lot of noise as mins mentioned, or disturbs people or the environment in some other way then there could be non-aviation, state or local laws that would apply.)

On the stall speed: this craft can hover, like a helicopter can. Is the regulation you use appliccable to ultralight helicopters as well?

@Koyovis AFAIK, if something meets the definition in 103.1 it's an ultralight. There are no separate regulations for ultralight aircraft/helicopters/powered paragliders/whatever. (Part 103 helicopters apparently exist, by the way.) For all I know the stall speed thing is a non-issue, I just mentioned it because it's the only 103.1 requirement that isn't addressed in the information from Zapata.

One slight problem... Zapata says the Flyboard carries 6.8 gallons of fuel. Max for part 103 is 5 gallons.
Reducing fuel storage to 5 gallons will give you maybe 8 minutes of flight time - that's a thirsty puppy.
You might want to look at a Mosquito ultralight helicopter - it has a 1 hour flight time and is fully part 103 compliant. Plus, if it loses power,  you have a decent chance of auto'ing down in one piece. If that thing loses power, you're going to have a very bad day.

It's correct that the Flyboard Air EXP carries more than 5 gallons, but the Flyboard Air UL carries 4.83 in order to comply with part 103. That reduces flight time from 10 to 6 minutes so I'd say they're both thirsty puppies!

It's likely that because it's controlled in a manner which is very different from the aircraft contemplated by the existing rules, that it would fall into a new category.
Consider the separate category for powered lift (harriers, etc).
Legislation always lags behind innovation, and this has "new rules" written all over it.
Also, please consider the above post from a due diligence perspective, which is to be conservative with risks (when evaluating, err on the side of "this will fail")


text:
What is the minimum recommended cruising altitude to avoid bird strikes?
I was surprised to see geese flying some 200 ft above my aircraft at 2000 ft MSL.
I obviously would like to avoid them but I am based too close to the New York City Class B, where there are much bigger, heavier, and faster birds:)
In order to avoid having to get clearance from ATC, I need to fly under the layered minimum altitudes by a safe margin of the upside down cake of the class B airspace.
What is the minimum recommended cruising altitude to avoid bird strikes?

Possible duplicate of What's the greatest height at which a bird has collided with an aircraft?

@RonBeyer: I don't think the other question provide elements related to a recommended altitude, it shows birds can be found very high, but it also implies that this is not frequent.

We have a California Condor Nesting Sanctuary near our airport and the sectional  asks pilots to maintain 3,000' terrain clearance. And from personal experience, I see hawks and vultures at the 2,000' level all the time so that seems like a reasonable altitude for avoiding large birds.

@RonBeyer I'm cooked then, it was at flight level 400, way above the ceiling for my Cessna:)

@0tyranny0poverty I guess the point is that no altitude is "safe" to avoid bird strikes, as your answer and comments say, the best way is awareness and caution, but you aren't going to reach a safe altitude in a single engine Cessna, especially working under a Class-B shelf.

Well geese have been known to fly at altitudes over 20,000ft.  Generally though altitudes of approx 4000ft AGL or more will keep you out of the way of most birds.

This question could be improved by specifying whether you are talking about a daytime or a nighttime flight.  It is rare to find birds above the top of the thermal convection during the day.  How high the thermal convection goes, would depend dramatically on the location and season, but in the eastern USA would usually not be much above 5K AGL.  Night migrants during migration season optimize their height to catch a tailwind and sometimes climb quite high.

AIM 7−4−1. Migratory Bird Activity
a. Bird strike risk increases because of bird migration during the months of March through April, and August through November.
b. The altitudes of migrating birds vary with winds aloft, weather fronts, terrain elevations, cloud conditions, and other environmental variables. While over 90 percent of the reported bird strikes occur at or below 3,000 feet AGL, strikes at higher altitudes are common during migration. Ducks and geese are frequently observed up to 7,000 feet AGL and pilots are cautioned to minimize en route flying at lower altitudes during migration.

I guess I have no choice but fly around the nyc class B

@0tyranny0poverty You just need to be extra vigilant during migration seasons and around locations where birds congregate e.g. ponds, dumps, fishing boats, etc. And maybe become familiar with the birds in your neighborhood. For example, if I see a lone turkey vulture cruising along, I probably won’t see more. But if it is circling, then chances are good that I will see a few more.

There are several bird sanctuaries in the NYC area, including Rikers Island, Clason Point Park, Hunts Point Riverside Park or Barretto Point Park. Below the shelf anywhere around The city is going to be possible bird strike area. US Airways 1549 hit the flock of geese around 2800 feet over the Bronx. So the birds had apparently entered the class B without clearance.


text:
What factors determine the DA(DH) and RVR/VIS of an ILS approach？
Look at the jeppesen chart of “VCBI-ILS Z Rwy 04 ”.
why the DAs are more than 200+RwyElev,and RVRs are more than 550m,
What factors determine the DA(DH) and RVR/VIS of an ILS approach？


text:
Should the transponder be set to 2000 then STB or STB then 2000?
For Airbus aircraft，after the engines are turned off，we set the transponder squawk code to 2000 and then the mode to STB.
Is it mandatory to set transponder 2000 and then STB？Can we set STB and then 2000? why?

Unless it is written in the airlines operation handbook or the planes operations manual I'd guess that's a just habit because in STB the transponder does not transmit anything and from your explanation I assume that the time between setting squark 2000 and turning to STB is too short that anybody could react on it.


text:
Is there any difference between Airbus and Boeing on yaw damper technology?
Modern aircraft have yaw damper systems installed. Is there any difference between the two big manufacturers on yaw damper systems, such as the location, etc.?
I have to do some research on this topic and haven't found any answers yet.

Interestingly, early on, autopilot systems like the Sperry C1 system used on the B-17 bomber were three axis with the A/P running the rudder to coordinate turns.  2 channel autopilots with a separate yaw damping system came along later due to the need for full time active yaw damping on swept wing jets when dutch roll issues came to the fore.  The system doesn't care what the AP is doing so much, it just does whatever it has to do to resist yaw excursions and keep the ball centered, AP on or not.

The difference is more between conventional flight controls vs. fly-by-wire rather than Airbus vs. Boeing.
Both Airbus and Boeing used separate yaw dampers in their non fly-by-wire aircraft, which applied their commands with separate actuators to the rudder control servos, without feedback to the mechanically linked rudder pedals in the flight deck. The exact details will differ for every aircraft of course, but the general design is quite similar. Consider e.g. the Airbus A310 and the Boeing 747:
Airbus A310:

The rudder is controlled by 3 actuators, which are commanded by a
single cable run from the rudder pedals. Rudder artificial feel is
provided by a spring-loaded rod.
Additional inputs to the rudder come from:

the rudder trim, and
the 2 yaw dampers.

[...]
The yaw damper ensures the following functions:

dutch roll damping
turn coordination which becomes active if sufficient control wheel deflection is applied [...]
yaw compensation in case of engine failure, provided the AP is engaged in CMD with SRS (takeoff) or GO AROUND mode annunciated on
FMA.

Yaw damper command is transmitted to the rudder but there is no
deflection feedback sent to the rudder pedals.

(Airbus A310 FCOM - Flight Controls - Rudder Control)
Boeing 747-400:

Yaw Dampers
Two independent yaw damper systems operate continously in flight to
improve airplane directional stability and provide turn coordination.
The upper and lower yaw damper actuators are powered by hydraulic
systems 3 and 2 respectively. Yaw damper inputs do not result in
rudder pedal motion.

(Boeing 747-400 FCOMv2 9.20.9 - Flight Controls - System Description)

On the other hand, fly-by-wire aircraft usually integrate the yaw damper logic into the fly-by-wire commands sent to the actuators. Consider e.g. the Airbus A320 and the Boeing 777:
Airbus A320:

Electrical Rudder Control
The yaw damping and turn coordination functions are automatic. The
ELACs compute yaw orders for coordinating turns and damping yaw
oscillations, and transmit them to the FACs.
Rudder Actuation
Three independent hydraulic servojacks, operating in parallel, actuate
the rudder. In automatic operation (yaw damping, turn coordination) a
green servo actuator drives all three servojacks. A yellow servo
actuator remains synchronized and takes over if there is a failure.
There is no feedback to the rudder pedals from the yaw damping and
turn coordination functions.

(Airbus A320 FCOM - Flight Controls - Description)
Boeing 777:

The yaw control law contains the usual functionality employed on other Boeing jetliners, such as the yaw
damper and rudder ratio changer (which compensates a rudder command as a function of airspeed).
However, the 777 FBW rudder control system has no separate actuators, linkages, and wiring for these
functions, as have been used in previous airplane models. Rather, the command for these functions are
calculated in the PFCs and included as part of the normal rudder command to the main rudder actuators.
This reduces weight, complexity, maintenance, and spares required to be stocked.

(Boeing B-777: Fly-By-Wire Flight Controls 11.8.2 - Yaw Control)


text:
Can you identify this engine?
I am studying Aircraft mechanics currently, and in our academy we have some aircraft engines and different types of them, and I need please your help to know the reference of one of them, and if it is possible the engine manual of it, because it is essential for us to study it deeply.

It appears to be a Rolls Royce Viper turbo jet engine made in 1966. The maker's mark (BSB) derives from Bristol Siddeley, formed from Armstrong Siddeley (the company that originally developed the engine) and Bristol Aero-engines. Bristol Siddeley were later taken over by Rolls Royce.

Source: Wikipedia


text:
What units should use for velocity prediction equations?
I'm not an expert aviation but I'm designing my own RC Plane and I wanted to estimate the maximum velocity using equations (15-20) from this paper. Honestly, I am not quite sure on the units to use for the Clmin equation and the Vmax equation, because when I try to apply the equations I end up with unreasonably high velocities.
The units I've been using for the parameters are:
$[W]=\text{lb}$
$[\rho]=\text{lb}/\text{ft}^3$
$[S]=\text{ft}^2$
$[V]=\text{ft}/s$
$[P_\text{max}]=\text{Watts}$

Watts are metric units, everything else is imperial... Choose one or the other system but don't mix them... Personally I'll go with N, kg, m, s

should I use Horsepower for the Power?

In principle it does not matter which system of units you choose to use as long as you are consistent. It is however always easier to do everything in SI units and then convert the result of your calculation into whatever unit system you prefer. Using inconsistent units has resulted in many accidents including the crash of the Mars Climate Orbiter. That is why NASA is using SI units now:

Public Law 94-168, §2 requires use of the International System of Units for measurement in U.S. Government programs, "except where impractical." That requirement is reflected in NASA policy, NPD 7120.4. That measurement system, formally known as the "SI" system," after its name in French, Systeme Internationale, is almost universally used by all countries except the United States. Use of SI measurement is growing in importance for international trade.

In your case, the mistake is using Watts for power, which is inconsistent with the remaining units you chose:
$$ 1 \, \text{W} = 1 \, \text{kg} \, \text{m}^2 \, \text{s}^{-3} \neq 1 \, \text{lbm} \; \text{ft}^2 \, \text{s}^{-3} $$
If you want to use imperial units, you need to convert your power to $ \text{lbm} \; \text{ft}^2 \, \text{s}^{-3} $, which can also be expressed in units $ \text{lbf} \; \text{ft} \, \text{s}^{-1} $ where lbf is the pound-force (lbm is the pound-mass)1. Horsepower is different again and I would advise against using it because there are many different definitions, the most common one being that 1 horsepower is equal to $ 550 \, \text{lbf} \; \text{ft} \, \text{s}^{-1} $.
If you stick to SI units (kilogram, meter and second), you can use Watts for power and you will get your velocity in meters per second, which is easy to convert to any unit for speed you want.

1 $ 1 \, \text{lbf} \approx 32 \, \text{lbm} \; \text{ft} \, \text{s}^{-1} $ (Thanks to Michael Seifert for the clarification), see also this question on engineering.SE.

Concerning the unit of $\text{lb}\cdot \text{ ft}^2 \cdot \text{s}^{-3}$: a pound can be either a unit of mass or of force.  The "pound-mass" (lbm) is related to the "pound-force" (lbf) by $1 \text{ lbf} = g(1 \text{ lbm}) = 32 \text{ lbm}\cdot\text{ft}/\text{s}^2$.  So the imperial unit of power is more commonly expressed as $\text{lbf} \cdot \text{ft}/\text{s}$, which is dimensionally equivalent to $\text{lbm}\cdot \text{ ft}^2 \cdot \text{s}^{-3}$.

More broadly, this nonsense about two dimensionally inconsistent units having the same name is a compelling reason to avoid Imperial and just use SI units in the first place.

@MichaelSeifert Yeah, this pound-force has always confused me, thanks for the explanation. I agree with your conclusion: just use SI ;)


text:
How much does a 3 day cabin crew recurrent training cost?
Airlines are getting rid of instructors. How are they carrying out recurrent training and how much does it cost the crew?

The airline will bring in contract instructors to do the training (yes there are freelancers out there that do that sort of thing as contractors).
If they don't have the number of active crews to justify full time instructors because they only need someone for 5 or 10 days a month, that's the normal alternative.
The airline will still be paying the contractors they hire for the training so the cost to the crew will still be zero.


text:
Does the civil industry use the CAN FD bus?
Does anyone know if commercial aircraft are using the CAN FD communication bus?

It's hard to prove a negative, but I would say the answer is no for the following reasons:

In all my years in the avionics industry, I've never heard of anyone
even discussing the possibility (though I realize CAN FD is fairly
recent).
There are numerous manufacturers that use ARINC 825 which is an
adaptation of the basic CAN.
The AEEC has no
active committee working to update ARINC 825 to include support for
CAN FD which would be the normal process before it would be used in
avionics.

Avionics suppliers just can't decide to use a new databus.  Their equipment has to talk to equipment from other manufacturers and if you don't have a defined protocol and agreement between suppliers it just won't work.

ARINC 825 2018 Supplement 4 describes supported extensions for capabilities derived from CAN FD such as increased data rates (up to 4mbps) and 64byte payload sizes.


text:
Does the Flight Acquisition Unit on the Boeing 737 NG stream data?
I noticed on B737-800 entering the flight deck on the right side there is a port Data Loader and I think this is the Flight Acquisition Unit.
Is this port used exclusively for loading data to FMCs etc or it can also read data?
Does the aircraft "stream" data through this port? If yes, which protocol is used?
I am trying to understand if it works like OBD-II we have in cars. Obviously not exactly the same, but is it similar? Does it stream live data?


text:
Could you start both A320 engines with APU BLEED on?
I've seen some procedures where one engine is started with APU bleed, then APU Bleed is turned off to avoid ingestion of non-desired gases and then the second engine is started with ENG 1 Bleed on.
However, my question would be: Would it be possible to start BOTH engines with the XBLEED Valve in the AUTO position, APU BLEED ON and assuming there are no critical issues (fire, ...)?
To sum up: XBLEED AUTO, APU BLEED ON. Could you start both A320 engines with that configuration?

Dunno about the A320, but with very similar engines the 737 absolutely can start both motors like that.

On a footnote here, the "ingestion of non-desided gases" is a minor issue. Even though the APU intake would ingest some fumes from the engines, no bleed air is fed to the AC PACKs while the engine start selector is set to start. So the fumes would not enter the cabin.

I have been flying A320s for over two years now and this is the first time I am hearing this. Normally the engines are always started using the APU with its bleed providing air for the air starter motor. So, in normal operations, cross bleed is set at AUTO, APU bleed is turned on and both the engines are started with the help of the APU. Once the engines are started APU bleed is turned off and the APU is shut down. Below is the after start flow of the A320. The green shows the flow of the pilot who starts the engines. As you can see, once the engine mode selector is set to norm, APU is shut down.

The procedure you are talking of is called a cross bleed start which is used when the APU is inoperative for the flight. It is also used by airlines that enforces one engine taxi to save fuel. When APU is not operational, we start one of the engines using an external air source. Once the engine is started the external unit is removed and then to start the next engine we use the bleed from the already started engine. To do that we open the cross bleed and increase the thrust on the running engine until the bleed pressure is sufficient enough for the second engine start up. As soon as both the engines are running we set the cross bleed to AUTO and revert back to normal operation.
As I have told before, I have never heard of any concerns regarding harmful gases coming of the APU bleed. Back when I used to fly Dash 8s we had issues in some of our airplanes smelling of burning oil once engine bleeds are turned on after start up. The airline consulted with Bombardier and they sent a bulletin which said that sometimes oil drippings from engine bearings can fall onto the bleed ducts. Because the duct temperature tends to be quite high, the oil burns off and the smell goes into the cabin. According to them, the lubrication oil used in airplanes is not hazardous or poisonous to human beings. I would say, it applies to all transport category airplanes with gas turbine engines because nearly all of them uses the same type of oil.

It's not a surprise that manufacturers claim their products to be safe, but the toxicity of oil fumes should be regarded as disputed. See https://en.wikipedia.org/wiki/Fume_event .

@bogl The wiki article you referred here has an event that happened in the 50s as an evidence that turbo oil can be harmful. How long it has been since then? I am pretty sure a lot of things have changed.

I didn't claim there is evidence in the article. I only referred to it to link the issue to its name, that is 'fume event'. It has gathered significant media attention in the past couple of years. Further to that, I am not saying it is toxic or not toxic - because I sincerely don't know. The point is, that the issue is disputed! https://www.google.com/search?channel=fs&client=ubuntu&q=aircraft+fume+event

@AnasMaaz, but this article is recent and quite well researched and clearly indicates that oil heat decomposition products and certain oil additives are toxic (so is hydraulic fluid, which also sometimes causes fume events, but this accident is blamed on oil).

@Jan Hudec The thing is we have been flying for quite a long time now and the amount of evidence we have for such events is too less to come to a conclusion. At the moment all I can say is that it is completely safe.

Yes you can, and at least some five years ago it was the A320 standard procedure.

I don't disagree with your answer, but could you maybe add some more details (have you personally done this?) or a source (FCOM, etc.)?

@Bianfable you're right. I should have. Though I don't have A320 FCOM anymore. I flew the A320 for a year, and we always started both engines with APU.


text:
Why should the dust protection on the Mi-17 helicopter not be used above 3000m?
The Russian Mil Mi-17 helicopters have a dust protection device fitted at engine intake (see the rounded part in front of the intake):

Its checklist gives an altitude limitation from sea level to 3000m AMSL. Why is the Mi-17 dust protection device not permitted to be used above 3000m?

https://aviation.stackexchange.com/questions/82532/why-do-the-intake-of-some-helicopters-have-a-bump-body, which notes that such systems reduce performance.  If so, I wouldn't want to need to use it above 3000m MSL.

Anything that blocks or restricts flow to the engines will hamper performance.
At low altitude, the aircraft has sufficient performance margin that this is acceptable.  However, engine performance drops as altitude increases.
They faced the decision:

Have no dust protection system -- the aircraft will have the best performance at all altitudes, but will not be able to operate in dusty environments without damaging the engines.

Always have the dust protection system -- operate in dusty environment, but suffer the performance penalty and limit the aircraft's ceiling and altitude performance.

Make the dust protection system removable and placard the aircraft that it shall not be operated at high altitude when the dust protection is installed.

To maximize operational flexibility, they went with 3).


text:
What is name of this part on a helicopter?
What is name of this highlighted part of helicopter and what is its use?

Guessing angle-of-attack sensor combined with sideslip sensor, i.e. a vane that pivots in 2 axes?

According to this cutaway it is called "low speed omnidirectional air data sensor system probe". Albeit overcomplicated, the name should be anyway self explaining.
The helicopter in the picture is a Bell AH-1 Cobra and, funnily enough, those kind of sensors are sometimes called cobra probe due to their shape.

Is it possible to know whether it is a pivoting vane (possibly in both pitch and yaw), or is fixed?

The round tail fin assembly tells you it's an articulating vane that weathervanes. It would almost certainly have freedom of movement vertically and laterally, allowing precise speed measurements at a range of yaw and pitch angles.


text:
Did Biplane pilots turn off their engines before landing?
I read or heard one time that radial engine biplane engines were turned off just before touchdown because if the throttle was used, the torque could cause the wing of the plane to contact the ground and a lot of pilots were killed just landing biplanes. Is that true?

Welcome to Stack Exchange! This website is for posting questions and answers; do you have a question? I don't see what your question is.

While turning off the engine was (sort of) a fact, it had nothing to do with the airplanes being biplanes.

Did Biplane pilots turn off their engines before landing?

Sometimes--
Rotary engines had rather primitive throttle arrangements, so a pilot would often use a "coup" button or "blip" switch, mounted on the control stick, to "blip" the ignition on and off and give partial power.
However, an alternative technique was recommended for some rotary engines:

By 1918 a Clerget handbook advised maintaining all necessary control
  by using the fuel and air controls, and starting and stopping the
  engine by turning the fuel on and off. The recommended landing
  procedure involved shutting off the fuel using the fuel lever, while
  leaving the blip switch on. The windmilling propeller made the engine
  continue to spin without delivering any power as the aircraft
  descended. It was important to leave the ignition on to allow the
  spark plugs to continue to spark and keep them from oiling up, so that
  the engine could (if all went well) be restarted simply by re-opening
  the fuel valve. Pilots were advised to not use an ignition cut out
  switch, as it would eventually damage the engine.[7]
Pilots of surviving or reproduction aircraft fitted with rotary
  engines still find that the blip switch is useful while landing, as it
  provides a more reliable, quicker way to initiate power if needed,
  rather than risk a sudden engine stall, or the failure of a
  windmilling engine to restart at the worst possible moment.

(Source: https://en.wikipedia.org/wiki/Rotary_engine#Monosoupape_rotaries, quoting Nahum, Andrew (1999). The Rotary Aero Engine. NMSI Trading Ltd. pp. 44–45. ISBN 1-900747-12-X.)
The key here is "rotary", not "radial", and regardless of which technique was used, the concern wasn't to avoid a hazard from engine torque or other related effects per se, but just to control the power output of the engine to allow the aircraft to descend for landing.
Of course, at other times, such as when taking off, the gyroscopic reaction to the whirling mass of the engine was a very significant factor while flying with a rotary engine, and the low ground clearance of the lower wingtips of most biplanes wouldn't have helped matters.
You can read much more here--
How are rotary engines controlled?
https://en.wikipedia.org/wiki/Rotary_engine
http://www.theaerodrome.com/forum/showthread.php?t=17121
http://www.theaerodrome.com/forum/showthread.php?t=2302&page=1&pp=10

@ymb1-- good point-- the question is essentially a duplicate, except that it is so overly broad.  Anyway I will add the link you supplied.


text:
Why does the A320 IR2 MEL OPS procedure suggest to switch to ADR3?
The A320 34-10-05B IR2 MEL OPS procedure additionally calls for Air Data switching to FO3.
When ADR 2 is working fine why replace its data on the FO PFD when only IR data is lost?

Probably a precaution since it's an ADIRS unit that computes AD and IR data?

Yes but each part is separate. IR for inertial and ADR air for data.
If that were the case why have a separate switching for inertial and air data and why not have the ECAM for IR fault also ask for air data switching. Don’t see the point.

Even if ADR and IR perform two independent and separate functions, they are the same hardware object (example). A IR2 malfunction is a malfunction: you do not know, in general, what is causing the problem of IR2 and it can possibly be a common problem (maybe undetected on ground by the ADR2).
For example, in the ADIRS Honeywell manual you can find in the IR FAULT chapter:

The ADR function of the affected ADIRU may possibly still work, even
if the IR function is not working. Cross comparing air data
information gives the pilot some idea of the validity of the ADR data
for that ADIRU.

So you have no guarantee that if IR2 is faulty, then ADR2 works. This is why, for precaution, you switch F/O on ADR3.


text:
What kind of horizontal stabilizer does a Boeing 737 have?
Does this plane have fixed horizontal stabilizers and elevators or has it been fixed with stabilators instead?

A Boeing 737 has a movable horizontal stabilizer for pitch trim with elevators for pitch control (also known as a THS – Trimmable Horizontal Stabilizer). This is true for all 737 variants, including the MAX. Note: The term stabilator is typically used when the entire tail is rotated for pitch control, like e.g. on a Piper Cherokee.
The following image (taken from this question) shows the horizontal stabilizer (the black arc indicates the range of movement of the whole stabilizer):

From the 737 NG FCOMv2 9.20.7 (Flight Controls - System Description):

Pitch Control
The pitch control surfaces consist of hydraulically powered elevators and an
  electrically powered stabilizer. The elevators are controlled by forward or aft
  movement of the control column. The stabilizer is controlled by autopilot trim or
  manual trim.
Elevators
The elevators provide pitch control around the airplane’s lateral axis. The
  elevators are positioned by the pilots’ control columns. The A and B FLT
  CONTROL switches control hydraulic shutoff valves for the elevators.
  Cables connect the pilots’ control columns to elevator power control units (PCUs)
  which are powered by hydraulic system A and B. The elevators are interconnected
  by a torque tube. With loss of hydraulic system A and B the elevators can be
  mechanically positioned by forward or aft movement of the pilots’ control
  columns. Control forces are higher due to friction and aerodynamic loads.
[...]
Stabilizer
The horizontal stabilizer is positioned by a single electric trim motor controlled
  through either the stab trim switches on the control wheel or autopilot trim. The
  stabilizer may also be positioned by manually rotating the stabilizer trim wheel.
Stabilizer Trim
Stabilizer trim switches on each control wheel actuate the electric trim motor
  through the main electric stabilizer trim circuit when the airplane is flown
  manually. With the autopilot engaged, stabilizer trim is accomplished through the
  autopilot stabilizer trim circuit. The main electric and autopilot stabilizer trim
  have two speed modes: high speed with flaps extended and low speed with flaps
  retracted. If the autopilot is engaged, actuating either pair of stabilizer trim
  switches automatically disengages the autopilot. The stabilizer trim wheels rotate
  whenever electric stabilizer trim is actuated.

Thanks so much for this elaborate answer. It's very clear and to the point.

The trim system is pretty much a bigger version of a Super Cub's or Cessna 180's, with an electric motor added.

The B737 has an all moveable stabiliser. It has fully irreversible hydraulic flight controls, with Q-feel loading in pitch. The neutral point of the elevator moves with stabiliser angle, and the stab provides pitch trim and pitch column zero feel.
When the stabiliser is activated, it sets to a new position and moves the trim point of the column as well. So if the pilot holds the stick and he selects the trim button, they will feel the holding force increase.
The stabiliser has a large area and therefore large authority on the aircraft trim. It can be very hard to overcome the stabiliser trim setting by trying to deflect the pilot flight control column.


text:
Why would a plane perform 360° turns to balance fuel on the ground?
Why would a plane need to do a couple of 360’ turns on the taxi way/run-up area. The crew were given clearance to taxi and hold. They told the tower they needed to turn the plane 360’ and were cleared. When they asked for a second 360  they said they were trying to resolve a fuel imbalance issue. I think this was a Gulfstream. What was going on with this.

I think it's more likely that the airplane was flying and did a 360 to buy time to balance their fuel, I can't see any reason why they'd do it on the ground.

Just curious, what clues in the question lead you to believe the airplane was actually flying?

@GdD You won't transfer fuel by gravity in the air by a coordinated turn.  You would have to slip or skid the turn to create a "downhill slope" for the fuel, in which case there's no point in turning anyway.  You just impart a bit of skid with rudder while flying along your normal track. In any case, if there's an imbalance in flight and the power transfer system is inop, you just open the gravity transfer valve and it will equalize itself without doing anything if the ball/brick is centered. Skidding would speed it up a bit, but it isn't necessary.

I wasn't suggesting that @JohnK, they'd pump fuel, the reason for a 360 might be to hold a position above the ground.

Oh Ok that makes sense.

They probably had a fuel imbalance that was marginally outside the limit for takeoff, and perhaps the powered fuel transfer system was unserviceable and was deferred.  Typically there is a gravity transfer capability by just opening a transfer valve and letting fuel run across on its own with the imbalance present, or by skidding slightly.
If they were say 50 or 100lbs above the max lateral imbalance for departure, a spin around on the ramp a couple times with the gravity transfer valve open so that some fuel "sloshes" through the gravity transfer line to the opposite tank, due to the centrifugal force of the turn, may have been enough to get the value in limits.

As to how such a fuel imbalance could arise...one candidate would be if they were sitting in a ground hold for a while with the APU running, as it will only draw fuel from one side of the plane.

Another candidate is the increasingly popular practice of taxiing with only one engine running to save fuel. If the fuel feed for each engine only draws from one wing tank it'll only run down the fuel on one side of the plane.


text:
What is the maximum number of infants allowed on a flight?
Is there any authority regulation which limits the maximum number of infants allowed on a flight?
Some airlines set a limit at 10% of total seats, some airlines have a different limit. Normally, the number of extra oxygen masks is much higher than 10%.

You're specifically asking about lap children, right? Rather than include kids under 2 who have their own seat.

Welcome to Aviation.SE! A good first question, if you haven’t already done so I would recommend taking the tour and reading the [help].

Since the question was only referring to infants, my answer treats them as a subgroup of SCPs (Special Categories of Passengers). SCPs are Persons with Reduced Mobility (PRMs), infants and unaccompanied children, deportees, inadmissible passengers, or prisoners in custody.
EASA's CRD containing the comments received to NPA 2014-01 has the following statement regarding SCP in chapter 2.5:

The Agency’s NPA did not establish a maximum number of SCPs on board an aircraft. The NPA proposed to clarify the existing AMC (AMC1 CAT.OP.MPA.155(b)) relating to the operator’s procedures that the number and subcategories of SCPs should not exceed the number of passengers capable of assisting them in case of an emergency.
In addition, the NPA proposed a new GM (GM4CAT.OP.MPA.155(b)) that clarifies  that  those passengers, who could assist the SCPs in case of an emergency, should not have another responsibility on board.

It's the closest I could find on this topic. Read the whole NPA, as it is a generally interesting read on SCP considerations (for example if using the word passengers or persons is more adequate to refer to the individuals on a plane).
Essentially, the CRD goes on stating that:

The  AMC  referring to operators’ procedures to establish the maximum number of SCPs has already been adopted by the Agency in 2012 and is based on the JAA Guidance Material.
Following the recommendations from the TÜV Rheinland study, the Agency does not propose any additional limits.
This  CRD  proposes a new  GM  (GM4  CAT.OP.MPA.155(b))stating that  a passenger capable  of assisting in case of an emergency means a passenger who is not an SCP and has no other role or private  responsibility  that  would  prevent  him/her  from  assisting  the SCP.  For  example,  an  adult travelling alone has no other role or private responsibility, unlike a family travelling with children.

In the US, I would assume that due to legal constraints originating on discriminatory aspects, there is also no hard limit as to how many infants are allowed on a flight, and the decision is left with the airline association and the NAA, with existing physical and safety constraints.

I know that SLC is Self-Loading Cargo. You define PRM can you also define SCP for those of us who are not pilots?

@FreeMan: I've amended my answer with the spelled out acronym: SCP refers to Special Categories of Passengers.

Apart from oxygen masks...another consideration is the number of infant life-vest. Typically airlines will x number of infant life vest and the reservation system will not confirm infants above this number. There can be requests made and some airlines may load extra vests on demand. In my airline we typically see a spike in infants traveling during festivals so we increase the number of infant life vests on certain sectors from 10 to 15 or 20.


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How are the wing control surfaces configured for takeoff?
For regular aircraft (i.e. a typical airliner), how are the wings configured before or during takeoff? Are there any adjustments needed for climb?

What kind of adjustments are you thinking of? Do you mean do any of the surfaces move? Or does someone go out with a spanner to adjust parts? Also what procedures for the wings?

How is your question different from these old ones? https://aviation.stackexchange.com/q/9183/1467
https://aviation.stackexchange.com/q/1449/1467


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At what operating temperatures would the Trent 900 need to operate, to equate to the efficiency of current geared turbofans?
What is the minimum optimum operating temperature (in degrees Celsius) that would be required for the Trent 900 to operate at, for it to attain the level of fuel efficiency of current geared turbo fans?

"What is the minimum optimum operating temperature that would be required for the Trent 900 to operate at, for it to attain the level of fuel efficiency of current geared turbo fans?" Do you know which is the "fuel efficiency" of the Trent 900 and/or the current GTFs or are you just asking for a "better efficiency" just for the fun?

There will be different answers for different operating points. I doubt anyone here will give you a good answer without major research. If you really want to know, get a good engine simulation software, model both engines and then tweak the parameters until you get the answer you want.


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Is hypersonic flight possible with a Busemann's Biplane?
The modifications to the Busemann's Biplane design reported in this article have proven that it is possible to design a modified Busemann's Biplane wing design that actually produces lift at supersonic speeds. Is it then possible to fly the same plane at hypersonic speeds of say Mach 10 (and ground speeds of above 10,000 km/h)? What are the other technical limitations?
The engines can achieve it and the air frame could be cooled, is noise once again an impediment at those speeds? Are there any effects on the human body for flying that fast given gradual acceleration? What about fuel, is it fuel efficient? Do we have a type of fuel that could power today's engine designs at those speeds given the complexities of using hydrogen?

Why Mach 10? Would Mach 3 or 4 not be enough? See here and here and here for the complications which arise at higher speeds.
The linked article is full of misrepresentations - of course will the Busemann biplane create lift at sub-supersonic speed (what is that, anyway? I read it as subsonic speed). Give it a proper subsonic airfoil and it will do so just like any other biplane, and even with the trapezoidal airfoil it will do so, albeit with more drag. It will also create lift and drag if operated at a proper angle of attack below its design speed (maybe that is meant by sub-supersonic speed, but the author never clarifies that).

Busemann biplane as shown in the linked article (source)
Did you note that the shock lines in the graph only occur between the two wings? That means that on the outer surfaces you will only see ambient pressure. The pressures internally cancel each other, and the result is that the Busemann biplane will not create any lift at its supersonic design speed in order to do its magic. As soon as it starts to create lift, it needs to create shock waves on the outside which will invariably be audible. That the researchers at Tohoku claim them to be 85% weaker is easy if they omit to mention to what they compare it.
Now look at the bulky passenger compartment on the top wing. This is clearly not optimised for supersonic travel (please compare it to the fuselages of the Concorde or the Tu-144 to get an idea what works).
No, this article is just utter nonsense.

Most things are 85+% quieter than the Tunguska event.

85% of all statistics are made up... oh, never mind.

Sorry, this answer is woefully out of date and does not even understand why the Space Shuttle had a blunt nose.

@RobertDiGiovanni Recent Mach 5-7 scramjet test vehicles have all had blunt noses, for exactly the same reason as the Shuttle. The SR-71 maxed out around 1,000 mph slower.

Yes, they have no other choice due to thermal effects, which also makes propulsion problematic.  It may work, being cost effective for commercial application may be another story.

@PeterKämpf You need to remember the distinction between a wing and a whole aeroplane. The wing does not necessarily have a blunt nose (though ISTR the papers linked in my answer include some that do - I recommend you read them!), but the fuselage does.

The idea is certainly valid.
Busemann's original design could not generate lift, but modern variations can. Here are just three recent papers on how it can be done:

Kusunose, Matsushima and Maruyama. (2011). "Supersonic biplane — A review". Progress in Aerospace Sciences 47. pp.53–87.
Wu, Jamieson and Wang. (2012). "Adjoint based aerodynamic optimization of supersonic biplane airfoils". Journal of Aircraft, Vol.49, No.3. May-June 2012. pp.802 ff.
Ma, Wang, Wu and Ye. (2020). "Avoiding Choked Flow and Flow Hysteresis of Busemann Biplane by Stagger Approach" (paywalled). Journal of Aircraft, Volume 57, Number 3, May 2020.

Wave drag has two causes, one due to the bulk or form of the plane and the other due to the lift generated.
The Busemann concept can eliminate form shock drag but not that due to lift. Busemann's original geometry eliminated all wave drag and hence also lift. Modern Busemann type designs can create lift, with its associated shockwave, while still eliminating much or all of the form drag, thus achieving considerable improvements in efficiency over conventional designs. They can also allow adequate performance over a range of speeds and angles of attack. The three papers linked explore different ways of achieving all this; some use variable geometry.
But Mach 10 cruise is less likely. We do not yet have the materials or airbreathing engines to achieve even half that for sustained flight. Moreover Mach 10 cruise requires extreme altitudes and it can be more economic to go for a conventionally-winged ballistic/suborbital spaceplane which maintains its cruise segment of the journey in empty space. The acceleration of any airborne vehicle can be limited to a level comfortable for the particular passengers, although varying gee levels can affect the economics.
Whichever approach is taken, to avoid overheating at hypersonic speeds above Mach 5 the fuselage nose would have to be bluff like the Space Shuttle rather than pointed like Concorde.
On engines and fuels, the only airbreathing technologies in the offing for these speeds are the scramjet and the airbreathing rocket (principally SABRE). For different technical reasons, both these are likely to be restricted to hydrogen fuel.

Interesting to note it may work holding the "wings" at 0 AOA and using the fuselage as the lifting surface at supersonic speeds.  At subsonic speeds the wings could be used conventionally.  With the amount of thrust required, one could consider vectoring it up too.

Not a bad idea, that. Certainly better than the concept, studied during XB-70 Valkyrie development, for detachable outer wing sections which dropped away for supersonic cruise!

Dropped or just tilted down? Imagine landing that thing without the outer section of the wings, approach and landing speeds would be insane...

@Jpe61 The final version tilted them down, where they enhanced the waverider effect. But landing with empty fuel tanks is less stressful on the wings than takeoff at full laden weight. For more see https://en.wikipedia.org/wiki/North_American_XB-70_Valkyrie#WS-110A

Oh, now I see what you mean. The Valkyrie that actually flew never had jettisonable wingtips as I recalled, but one of the early proposals did. Interesting!


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Are there any planes that have ramjet or scramjet engines?
Are there any planes that have ramjet or scramjet engines? If so, how did they reach the required mach speed for the engines to start functioning?

Scramjets
There's only a couple flying examples. The early Soviet-born Kholod, the HyShot series, and the recent ISRO ATV all flew small scramjet engines, although none were designed for net thrust. By far the most [publicly] successful vehicles have been the X-43A and the X-51. Both were fully controllable, independent flight vehicles (rather than just engines hanging from rockets) that produced net thrust.
The NASA X-43A hypersonic research vehicle was dropped from a B-52, then boosted by a modified Pegasus booster, and briefly flew at Mach 9.6 with a hydrogen-fueled scramjet.

Credit: NASA

Credit: NASA
The Boeing X-51 Waverider hypersonic research vehicle was also dropped from a B-52, then boosted by a modified ATACMS (an Army surface-to-surface missile), and then flew at Mach 7 with a JP-7-fueled scramjet.

Credit: USAF
The DARPA Falcon program aimed to produce a hypersonic plane that could takeoff and land under its own power using a turbine-based combined cycle (TBCC) propulsion system (NASA TBCC overivew on NTRS). Two early designs, HTV-1 and HTV-2, were unpowered hypersonic gliders, the latter was launched by ICBMs (basically reentry vehicles, like the X-41 they were probably derived from). But the next two designs, HTV-3X Blackswift (YouTube) and the final Hypersonic Cruise Vehicle (HCV) (YouTube), would have been the first reusable, hypersonic CTOL aircraft. HCV would have been able to carry a few thousands pounds of payload (eg bombs, flight test equipment, cameras) very quickly across great distances. The [HTV-3X and HCV] program was eventually canceled, but other work continues on hypersonic weapons and vehicles.

Credit: DARPA
SR-72. Lockheed Martin is developing the SR-72, an unmanned Mach 6+ ISR (recon) platform. It's meant to fly completely under its own power using TBCC, taking off with turbines and transitioning to ramjet and finally scramjet operation. A key innovation was bridging the speed gap between where turbojets stop working well and where ramjets start working well, but LM declined to elaborate how they achieved that. Their design apparently builds upon their work on the Falcon program (see above). They said they'd fly a single-engined demo before moving to the twin-engined, full-scale vehicle.

Credit: Lockheed Martin
Other ambitious unflown designs include the LLNL Hypersoar and the X-30 National Aerospace Plane. Both were large (100-200 tonnes MTOW), hypersonic, intercontinental vehicles. The SSTO-capable X-30 could launch payloads to LEO as well as serve intercontinental passenger routes. Hypersoar was also envisioned to carry passengers in one variant, perform TSTO in another, and also perform global strike/recon in yet another.

X-30 NASP. Credit: Chad Slattery

Hypersoar. Source: FAS.
Ramjets
The Republic XF-103 Thunderwarrior was essentially an early attempt at TBCC in a manned interceptor. It was a very innovative design (Mach 3-5, no canopy, periscope, crew capsule, huge radar, and internal bays for 6 large missiles, precursors to the AIM-54) but never flew.

Source. Unknown.

Source with lots of great background.
There's a variety of ramjet powered missiles
Notable examples include...
... the Bomarc. Credit: Boeing.

... the GQM-163 Coyote, a notable supersonic, sea-skimming target. Credit: DR Kirk.

... the massive P-700 Granit Source and its relatives, the P-800 Oniks and the BrahMos.

... the RIM-8 Talos SAM. Credit: USN.

... and the Sea Dart, the Moskit, the Hughes/Raytheon AIM-152 (and related ACIMD), the Triple Target Terminator, and a bunch others. Some are antiship missiles (various trajectories), some are SAMs, and some are air to air missiles. All were rocket-boosted before the ramjet took over.
Obviously the most "terrifying" was the Supersonic Low Altitude Missile of Project Pluto fame, a nuclear-armed, nuclear-powered ramjet for the 100,000 mi cruise missile. Air was blown directly through the reactor, directly heating it for thrust. Its unshielded reactor would have propelled it to Mach 3+ at sea-level and irradiated targets below it before crashing into a final target, irradiating that as well. They successfully tested two engines before the program was cancelled in favor of ballistic missiles.
Here's a fun USAF-Convair vid from 1959 explaining the concept:

Credit: USG

I tried to reduce a bit the amount of scrolling needed to go through your answer. Please consider revising this answer further, particularly try to evaluate if all images need necessarily to be included or could be simple links.

Are you asking about current production or through out time?
The SR-71 was powered by a Pratt & Whitney J-58 engine which operated as a partial ram jet at high speeds. At lower speeds it operated like a turbojet changing its internal shape as it increased its speed. The SR-71 was capable of departure under its own power from the ground.
You can find a list of ramjet powered aircraft here however many are prototypes or test beds.
NASA's X-43 hyper sonic test bed is powered by a scram jet but that is unmanned. It is launched off a B-52 and connected to a pegasus booster to get it up to speed.

Current production and throughout time

René Lorin invented the ramjet in 1908, and his ideas were tried in flight for the first time from 1941 onwards. Different sizes were tried; below are two pictures, one of a Do-17 Z with a small Lorin engine (source) and below that one of a Do-217 with a much larger version (source). Speed increases of up to 150 km/h were recorded, but efficiency was poor. In all cases the piston engines were needed to take off and reach a speed at which the Lorin engines could be started.

If you accept a pulsejet as a kind of ram jet, the next manned, ramjet powered flight used a Heinkel 280. This plane was designed as a jet fighter but the engines were delayed. Therefore, in 1943 it was decided to try it out with pulsejets, two on each side. It had to be towed aloft and when the jets were started, their vibration shook the whole airplane apart. Wikipedia has the story differently; here the aircraft took on too much ice. The pilot could escape by using the newly-invented ejection seat; another first on that flight.
Later the same engine was used on the V1 "buzz bomb", and almost 30,000 were made between 1943 and 1945. They needed a catapult for take-off, so again none were able to fly all by themselves.
The next chapter happened in France, where René Leduc worked on ramjets from 1933 on, culminating in a range of designs. The Leduc 0.10 counts as the first airplane which flew solely powered by a ramjet in 1949. But those aircraft needed a launching aid as well; in the picture below it is a Sud-Est Languedoc carrying a Leduc 0.16 (source).

The final design of this series, the Leduc 0.22, was supposed to be a supersonic interceptor with a coaxial turbojet and ramjet, enabling it for independent operation. A similar propulsion system was chosen for the Nord 1500 Griffon interceptor prototype.
Meanwhile, in the USA, a ramjet was used for several unmanned long-range designs like the CIM-10 Bomarc, which was rocket-launched and capable of Mach 2.5 flight.
Based on work by the Austrian engineer Friedrich von Doblhoff, Hiller produced the YH-32 which used two ramjets mounted at the tips of the rotor blades for propulsion (below, source). This allowed to take off under ramjet power, but the limited speed of the rotor tips meant that the ramjets had poor efficiency.

Another notable use of ramjets is the Lockheed D-21 drone which was air launched as well, initially from a SR-71-derived M-21 and later from the B-52.
More recent designs using ramjets are long-range air-to-air missiles like the Meteor.
Moving over to scramjets, I can name only two unmanned designs which were tested, one the US X-43A experimental plane (left, source) and the other the Russian Ajax or Ayaks project (right, source).

CFD simulation of Hyper-X, which lead to the X-43A, and test model of the Russian Ajax/Ayaks hypersonic project

Slight addition. The MBDA Meteor is a ducted rocket rather than a pure ramjet.


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When is the autopilot engaged and disengaged?
When does the pilot engage the autopilot during a flight? And when does he disengage it - e.g. at what height - during the descent?

Welcome Adam. Your question is not bad, but has been likely already answered in: On modern commercial airliners, how much of the flight could be fully taken care of by the auto pilot?

It could be broken into sections for ease of reading and answering.

I am a B777 pilot. On departure, we generally hand fly until about 10,000' to keep up our skills. If the departure procedure is complicated, we will ask for the AP on at 200' so there is no chance of messing up the departure procedure.
On approach I typically disconnect the Autopilot at about 1500' AGL once visual contact is assured and I am inside the Final Approach fix.
If there is any chance of a possible missed approach due to weather, or other conflicting aircraft, on the runway or elsewhere, I will leave the AP on until there is virtually no chance of a missed approach.
Most of the pilots I fly with tend to do the same.


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Can a Kline-Fogleman airfoil be used for engine turbines?
I am currently working on the Kline-Fogleman airfoil for NACA 23015 variant at Reynolds number 208800 with the configuration of the backward facing step of 50% thickness at 50% chord on the bottom surface of the airfoil. The KF airfoil gives higher L/D ratios for all pre-stall AOA and it also stalls at a slighty higher angle than conventional variant of the same series. The highest L/D value from Ansys Fluent was 22.45 at 9deg AOA as compared to conventional NACA 23015 at 9deg AOA being 19.74. I want to know if the KF variant of this airfoil profile can be used in turbine blades of a gas turbine engine and if it will produce higher power output or efficiency. Will it cause any problems due to turbulence and the number of blades and stages?
Also, are there any other applications of KF airfoils other than RC planes?

KF airfoils result from folding paper airplanes. In a way, most paper airplanes employ an airfoil with a stepped contour. At the scale of paper airplanes, this step possibly forces a laminar-turbulent boundary transition which might allow the airfoil to stall at a higher angle of attack.
A regular NACA 23015 has an optimum L/D of close to 50 at a Reynolds number of 200,000 and 9° angle of attack. Airfoiltools gives it as 48.5.
If you doubt the XFOIL data from Airfoiltools, NACA TR 824 gives essentially the same values using wind tunnel data, however, you need to pick Airfoiltool data at a higher Reynolds number to compare the two.
Since the L/D of the conventional airfoil is two an a half times better than what you calculated with ANSYS, I recommend that you review your results. And no, I would not recommend their use in turbines. The exposed corner at the step will face extreme heat loads and film cooling will probably fail there.


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How do pilots do a preflight inspection of tail-mounted engines?
The recently released first part of an Air Clips documentary of a Lufthansa Cargo MD-11 FRA-NBO flight shows in some detail the pilot carrying out the preflight visual inspection of the aircraft (from ~ 6:20 min). As in many other pilots' accounts of these 'walkarounds', the captain states one important aspect is the condition of the engine, including e.g. fan blades, intakes, checks for any foreign objects.
I understand that a major point of this visual inspection is to have the pilot-in-command getting a full picture of the aircraft condition before assuming the final responsibility for the flight. Obviously, the PIC cannot inspect a center tail-mounted engine like on the MD-11 by himself during the walkaround – so when or how is that inspection done for the center engine? I imagine that an inspection of the center engine is just as important as for the wing-mounted ones, prior to each takeoff.

I think it's rare for the pilots (not necessarily the PIC mind you) to do a full walk-around for each take-off. Usually one thorough one is done at the start of the day or before the new crew taking the aircraft, and an abbreviated one before each flight (engines are there, wheels aren't flat, tube's and safety items look good, etc).

This is a good question re: first-flight-of-the-day inspections though -- it's similar to the issues posed by the DC-9 series elevator system re: preflight.

In personal experience, I often look for a step ladder to inspect ground inaccessible engines and control surfaces and I have seen others look at their aircraft from a second story window.
My personal practice is to walk around the aircraft prior to every flight, not just at the first flight of the day, but I often had to make time for that. It may not be common practice. But that is an aside.
Here's the reality. If you are talking about the walk-around that the pilot or flight engineer might do, there are not up close inspections of the elevated engines and control surfaces. The walk-around will look for leaking oil, missing $1500 fasteners or whatever, but one is not checking the dipstick prior to engine start.
How are things inspected?  The manufacturer factors that into the design.
From an operational standpoint, the pilot(s), FE will look at performance during engine start, and make sure that the profile does not deviate from the expected temps, flow, etc.
In summary, if you can't get to it, it is not likely on the walk-around checklist.
Addendum.  At least one company is seeking FAA approval to utilize drones in aircraft maintenance inspection (not pre-flight) processes, which would reduce the cost and risks associated with scaffolding and lifts. As part of their application to FAA, there will be demonstration periods where aircraft are inspected with both methods, and results are shared with the FAA.

Nice to see an answer updated so long after the original posting. The drone idea sounds quite novel. Do you have any links or references to it?

Things change.  Someone upvoted, and I went back to look at my answer, and realized that there was a new development to share.  One company is ARR, as I recall.  mrodrone.net is a drone provider.  https://aviationweek.com/%5Bprimary-term%5D/aar-testing-drones-aircraft-inspection-miami   Also Austrian Airlines is trying drones for aircraft inspections as well.


text:
Can flight control computers detect and help to avoid pilot-induced oscillations?
With flight control technology available today like fly-by-wire, digitally augmented command inputs etc., I wonder whether any technical solution exists (or could possibly exist) to detect and counter the occurrence and dangers of pilot-induced oscillations (PIO).
Consider for example a low visibility final approach scenario:

Is it possible to detect an early onset of PIO and either augment pilot input to prevent further amplification of oscillations, or, less intrusive, give some (e.g. aural) warning to the pilot?
Also, a fully developed PIO situation might be detected and a warning triggered to increase awareness or prompt the pilots towards an eventual go around, similar to windshear warnings.

If feasible, do such technical mechanisms exist, or would they even be considered helpful by pilots at all? I imagine a critical part to solve would be not to override an actually intended control input by the pilot (that is within the usual safe envelope).

I would think that Airbus flight laws make PIO rather unlikely (the aircraft auto-trims to fly straight when you release the stick and the rather low sensitivity leads to rather atypical piloting technique of quick nudges with releasing the stick in between). But that's just one specific kind of FBW and I have no idea what the others do.

Actually the auto-trim is likely to make it more PIO prone if not designed properly. Often integrators are used to provide auto-trimming and this will introduce a lag in the system which will result in a system more susceptible to PIO. Careful design with a feed forward path is required to make it PIO resistant, the Gripen PIO accident was a problem with the early design and was easily corrected.

PIOs occur because the pilot senses a rapid movement and feels the urge to counter it. Key is the frequency of a few Hertz, so the pilot's reaction kicks in with a phase shift such that it promotes the movement, and low damping such that the oscillation amplitude grows over time. With a working FCS this kind of rapid movement should not happen.
However, once the FCS is not tuned to the situation, it can actually aggravate the situation and promote PIOs. The best known case is probably the 1992 crash landing of Tom Morgenfeld with the YF-22 prototype.

Here the gains in the FCS were set too high with afterburner on and gear up at slow speed.
Another case happened in the early phases of flight testing the V-22 Osprey. Here two of the three gyros in the control system were incorrectly wired and the pilot made a wrong control input.

So it is less the ability of the FCS to detect PIOs than its improved damping which never lets a rapid oscillation happen in the first place.

There have been some cases of PIO on Airbus A321 aircraft in certain conditions which then led to an update of flight control laws: http://www.tsb.gc.ca/eng/rapports-reports/aviation/2002/a02o0406/a02o0406.pdf

@CptReynolds: As I read it, in both cases icing changed the aerodynamics. With a clean aircraft this would not had happened.

Sorry but I consider this to be quite a misleading description on the root cause of PIOs. PIOs occur due to a lag in the feedback system so that the aircraft is not responding as the pilot expects it to. Rapid movement is not a problem per se, there are very agile combat aircraft that are PIO resistant. Also you can get low frequency PIO problems, e.g. a phugoid PIO, the space shuttle early design had a PIO problem on approach which wasn't rapid movement. Finally the YF-22 was a rate limit PIO and not the FCS gains. When it went into rate limit it introduced a lag leading to the PIO.

@Adrian: This was not meant to be a definition of what a PIO is. Did you read and comprehend the question? It seems the asker is keenly aware of what a PIO is and wonders about the role of the FCS in it.

I did read the question. However my comment was in reply to your response not to the original question. If it was not meant to be a definition of what a PIO is then why start with "PIOs occur because"? With it becoming the accepted answer I think a little more rigour in the answer is required and in my opinion some of the description related to PIOs is misleading.

Maybe, but it would require giving a special feedback to the pilot.
PIO occurs when the pilot input is lagging or leading the tendency the pilot is trying to correct, so the pilot ends up reinforcing it, instead of dampening the oscillation.
To break this loop there would need to be some kind of instrument or display that enabled the pilot to visualize the lag and also indicate how much control movement is necessary to correct it. Such a system would not be trivial to design.
One point of fly by wire systems is to make PIO unlikely because the pilot does not have to guess how much input will generate a certain result. The pilot just says, I want to fly level and the plane takes care of the rest. In other words the pilot does not need to guess how much to move the stick to level the plane. Thus, a fly by wire system should theoretically make what you are proposing unnecessary in the first place.

"To break this loop there would need to be some kind of instrument or display..." Or a pilot that realizes their mistake and stops making that mistake. A PIO only happens because the pilot "chases the ball", once you realize you are in that situation all you have to do is hold steady or go around.

Fly by wire systems don't make PIO unlikely by pure virtue of being FBW. If the FBW emulates behaviour of mechanical controls like the Boeing one, PIO is just as likely. The flight laws is what might prevent PIO.


text:
How often are heavy aircraft brake pads changed?
How often are Boeing 747-800 and Airbus A380 aircraft brake pads changed?

A technical question: Are they actually brake shoes, or are they brake pads? The difference being shoes are used in drum brakes, while pads are used on disk brakes. I can't imagine an aircraft (especially a heavy/super) using drum brakes since disk are much more effective. (Witness all modern cars using (at least) front disk brakes (since the fronts do about 75% of the braking), and the terrifying lack of braking available in muscle cars from the '60s-'70s where 4-wheel drums were standard.)

@FreeMan pretty sure they're all disc brakes on modern airliners.

It's more like "brake plates" like clutch plates.

The 747-800 uses carbon brakes made by the Goodrich company and according to this article,

The entire industry is leaning towards carbon brakes, Honeywell
  executives assert. Lufthansa Technik also sees a trend towards carbon
  brakes, as the mean time between repairs (MTBR) for carbon brakes is
  1,500-2,000 cycles, compared with some 1,100 cycles for steel brakes.

Also of note, take a look at this video on the testing of the brakes.